In a coal-fired boiler in the related art, for example, raw-material coal is input to a coal pulverizer, such as a vertical roller mill 10 shown in FIGS. 9 and 10, from a coal feeding pipe 14, and ground pulverized coal is used as fuel. Inside the vertical roller mill 10, a grinding roller 13, while rotating, revolves on a grinding table 12 installed in a casing 11 at a lower portion thereof.
The raw-material coal input to the vertical roller mill 10 is ground into pulverized coal by being crunched between the grinding table 12 and the grinding roller 13. Hot air jetted from a throat 15 disposed in the periphery of the grinding table 12 transports the pulverized coal by means of an airflow, while drying it, to a fixed-type sorter 20 disposed thereabove in the casing 11. Because gravitational sorting is performed at this time, in which large coarse particles having large particle sizes drop due to gravity and are returned onto the grinding table 12, the pulverized coal is repeatedly ground until a desired particle size is achieved.
After the primary sorting by means of the gravitational sorting, product pulverized-coal particles containing coarse particles are further sorted by the fixed-type sorter 20 disposed at the top of the grinding table 12. This type of sorter includes a rotating type and a type in which a fixed type and a rotating type are combined, in addition to the fixed-type sorter 20. Note that a rotating-type sorter performs sorting by means of collision with rotating blades and an inertial force, and it is known to have high sorting capability.
The pulverized coal transported by means of the airflow is dried by the hot air, and, in addition, it is sorted by passing through the fixed-type sorter 20. The sorted pulverized coal passes through a pulverized-coal exit 16 which communicates with the exterior above the casing 11 from the interior of the fixed-type sorter 20 and is transported to a boiler (not shown) by means of the airflow of transporting primary air.
The fixed-type sorter 20 is provided, at the top end of a cone 21, with numerous fixed-blade entrance windows 22 serving as openings at equal intervals in the circumferential direction. The fixed-blade entrance windows 22 are opening portions provided so as to penetrate through a wall surface that forms the cone 21 and serve as entrances and flow paths where the flow of the pulverized coal transported by the airflow (hereinafter, referred to as “solid-gas two-phase flow”) passes through to flow into the interior of the cone 21. Fixed blades 23 corresponding to the individual fixed-blade entrance windows 22 are attached to the inner wall of the cone 21.
An inner cylinder 24 that forms a wall surface facing the fixed-blade entrance windows 22 and the fixed blades 23 is provided on the inner side of the cone 21. In order to cause the solid-gas two-phase flow to spiral, all fixed blades 23 are attached with an inclination in the same direction, that is, having an inclined angle with respect to a line extending in the radial direction toward the axial center of the cone 21. Therefore, by increasing or decreasing the inclination angle of the fixed blades 23, the intensity of the spiral flow can also be changed in accordance with the degree of opening (angle) of the fixed blades 23, which makes it to possible to adjust the powder fineness for sorting.
Note that the bottom end of the cone 21 serves as a cone exit 25 from which the coarse particles sorted by the fixed-type sorter 20 are supplied onto the grinding table 12.
Because the fixed-type sorter 20 is inferior in terms of sorting precision in a coarse particle region, and an increased amount of coarse particles (coarse particles whose approximate sizes exceed 100 mesh, which adversely affect combustibility) is contained in the pulverized coal, this causes uncombusted components contained in combustion exhaust gas expelled from the boiler to increase.
In the fixed-type sorter 20, the solid-gas two-phase flow that passes through between adjacent fixed blades 23 from the fixed-blade entrance windows 22 centrifugally sorts pulverized coal particles into coarse particles and fine powder by means of a spiral flow. Subsequently, the fine powder that has a small particle size and low weight is swirled up by being carried by a reverse upward flow from below the cone 21, enters the inner side of the inner cylinder 24 from below the inner cylinder 24, and flows out to the exterior of the vertical roller mill 10 from the pulverized-coal exit 16. On the other hand, because the coarse particles having large particle sizes that have been centrifugally separated are heavy and cannot be carried by the flow entering the inner side of the inner cylinder 24 from below the inner cylinder 24, they reach the inner wall of the cone 21 and fall downward along the inner-wall surface of the cone 21 due to gravity. The coarse particles eventually fall onto the grinding table 12, to be ground again, from the cone exit 25 provided as an opening at the center of the lower portion of the cone 21.
In the related art with regard to vertical roller mills provided with a fixed-type sorter, in order to enhance the sorting capability for ground pulverized coal, it has been proposed to modify flat-plate fixed blades into wave-shaped vanes. With the wave-shaped vanes, even if coarse coal particles flow in from all entry angles when a mixed airflow spiraling upward together with primary air is taken into the spaces between the waved-shaped vanes of a fixed-type sorter, because collisions occur at airflow colliding portions of the wave-shaped vanes and sorting is performed, the sorting capability of the fixed-type sorter is enhanced (for example, Patent Literature 1).
In addition, with rotating-type sorters that have a high sorting capability, in order to further enhance the sorting capability thereof, two stages of rotating blades are disposed in the direction of the rotation axis, and lower-stage blades are inclined with respect to an outer circumferential wall of a rotor, or pivoting portions that can freely pivot are provided at tip portions of the rotating blades (for example, Patent Literature 2).